The present invention relates to an apparatus and a method for determining a deformation speed of an elastic member through which a driving force is transmitted from a driving element to a member to be actuated, and also to an actuator in which such an apparatus or a method is implemented.
In recent years, a robot including an articulated structure with a joint has been proposed in which a rotary driving force from a driving element, such as a motor or a hydraulic actuator, is transmitted through an elastic member, such as a spring, to a link connected by the joint (see JP 2008-055541 A; a corresponding US Patent Application, assigned to the same entity as of the present application, has been published under US 2008/0075561 A1). This type of articulated structure is called Serial Elastic Actuator or SEA. When the link collides with an obstacle or the like, the elastic member deforms so that the obstacle or the driving element can be protected and the robot can be controlled appropriately based upon a load applied to the link which may be determined from the amount of deformation of the elastic member as detected, or otherwise.
To appropriately control the load applied at the joint is to appropriately control the amount of deformation of the elastic member. In addition to the amount of deformation, an accurate deformation speed (i.e., time rate of deformation) of the elastic member may be determined and used for that purpose. For example, when the deformation speed is changing, it would be beneficial to know whether the deformation speed is increasing or decreasing, and how great is the deformation speed of the elastic member, because the magnitude of the deformation speed would be a significant factor to be considered in determination of the strategy for control over the motion of the link, such as the optimum direction and amount thereof.
In order to determine the deformation speed of the elastic member, several methods may be adopted for determination which may be made by handling (or manipulating) data in either of an analog form or a digital form. One known method for determining the amount of deformation as analog data uses a strain gauge. This method using a strain gauge would allow errors (e.g., due to ambient temperature variation) or electrical noises to be introduced much into the determined data, and thus could not yield sufficiently precise results. In contrast, the method of detecting displacement (i.e. deformation amount) by using an optical sensor is currently available without such a problem; i.e., the deformation speed can be determined with a high degree of precision. This method may use an encoder to thereby obtain digital data (quantized values) as outputs.
As is often the case with the method for determining the amount of deformation of the elastic member, as quantized values, it would be difficult to accurately estimate the deformation speed. The deformation speed may be obtained by dividing a difference between two positions at the present point and the preceding point in time (i.e., the amount of deformation) by a sampling period. If the positions are represented as quantized values, the speed exhibits 0 at most of the points in time, and exhibits very great computed speeds only at a limited number of points in time on calculation.
In order to approximate the values, which are discrete in time, of the deformation speed to continuously changing values, a low pass filter may be applied to the deformation speed, or a curve fitting to the amounts of deformation may be determined and differentiated to obtain a smooth curve of the time-varying deformation speed less subject to fluctuations. However, the values of the deformation speed obtained by these methods would involve a delay or time lag between the obtained and true values, and thus would result in poor responsiveness particularly when the elastic member starts deforming. Although the responsiveness would possibly be improved by reducing the number of data consulted back to the past (and used for calculation) for approximation, the data obtained by this approximation would become excessively responsive to a negligible amount of change in the amount of deformation, and disadvantageously the stability would be impaired, instead.
The present invention has been created in an attempt to eliminate the above disadvantages.
Illustrative, non-limiting embodiments of the present invention overcome the above disadvantages and other disadvantages not described above. Also, the present invention is not required to overcome the disadvantages described above, and an illustrative, non-limiting embodiment of the present invention may not overcome any of the problems described above.